This invention relates generally to temperature compensation circuits for oscillator circuits and, more particularly, to temperature compensation circuits for oscillator circuits having frequency responses that are parabolic with respect to temperature.
Many modern electronic circuits require high frequency oscillators which are capable of maintaining extremely stable frequency outputs over broad operating temperature ranges. While in the past it may have been satisfactory to provide oscillators achieving maximum frequencies of up to about 125 MHz at accuracies of about 10 to 100 parts per million or less, today it is often necessary to meet specifications of as low as 1.0 or even 0.1 parts per million at frequencies of 100-500 MHz or more.
Conventionally, uncompensated quartz crystal oscillators are capable of producing the maximum 125 MHz frequencies and 10 ppm accuracies described above, depending upon the cut of the crystal. Surface acoustic wave (SAW) devices can be used in lieu of the quartz crystals to produce the higher frequencies in the range of about 100 MHz to 500 MHz or more, at accuracies comparable to those produced with bulk wave quartz crystals. Compensating circuits, such as that described in U.S. Pat. No. 3,821,665, assigned to the same assignee as the present invention, can then be used to bring accuracy up to the desired 1.0 to 0.1 parts per million level.
Unfortunately, presently available temperature compensating circuits are generally complex and expensive to build and adjust. Many of these circuits produce an extremely undesirable power drain. U.S. Pat. No. 3,821,665, referred to above, describes one excellent temperature compensating circuit which obviates many of the above drawbacks. This circuit, however, is still somewhat complex and expensive since it utilizes a series of multiplying means corresponding to first, second and third order temperature variations generally exhibited by quartz crystals.
Since SAW devices and certain quartz crystals, such as BT cut crystals, exhibit almost purely parabolic frequency shifts with respect to temperature, the invention of a compensation circuit which corrects this type of shift in a simple and straightforward manner would constitute an important contribution to the art.